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A Nancy M. Albert, RN, MSN, CCNS, CCRN, CNA replies:

There are many systems available (some in clinical trials) that have the capability of noninvasive monitoring of cardiac output. Some require arterial or central venous catheter connections to the monitoring device and are not truly noninvasive. One type of noninvasive system is called impedance cardiography (ICG) or thoracic electrical bioimpedance. This system works by injecting a constant, low-grade (1–4 mA) electrical current into the chest to sense change in thoracic electrical impedance that is associated with change in blood flow in the aorta during the cardiac cycle. During systole, a beat-to-beat voltage drop is associated with increased blood volume and flow in the aorta. The monitor assesses electrocardiogram and thoracic impedance changes to calculate stroke volume.¹ Cardiac output is derived from the patient’s heart rate and stroke volume. To determine base impedance and changes in impedance, 2 sets of sensors are placed on the neck and 2 sets of sensors are placed on the thorax, 180° opposite each other. The outer electrodes on each sensor inject electrical current and the inner electrodes sense thoracic impedance. Each system uses an equation to calculate stroke volume from the impedance cardiogram, and the equation used will influence the accuracy of the hemodynamics calculated.² Therefore, it should not be assumed that ICG systems are alike and have similar accuracy to thermodilution cardiac output technique.

It is important to review current research on the agreement between ICG and thermodilution cardiac output of the system you plan to use. In the United States, two 510k systems cleared by the Food and Drug Administration are the BioZ ICG Monitor (CardioDynamics Inc, San Diego, Calif ) and the BioZ ICG Module (GE Medical Systems Information Technologies, Milwaukee, Wis). These monitors provide continuous, risk-free, accurate cardiac output readings in many patient populations, including postoperative cardiac patients,³ stable and acutely decompensated patients with chronic heart failure,^4–6 and patients requiring mechanical ventilation.⁷

Q What criteria do you use to determine noninvasive cardiac output monitoring versus thermodilution cardiac output monitoring?

A Thermodilution cardiac output monitoring requires pulmonary artery catheter insertion and monitoring, which has been associated with increased cost of care (catheter, insertion, nurse monitoring costs), morbidity (ventricular arrhythmias, infection, blood loss, thrombus), and mortality.^8,9 When determining which system to use to obtain cardiac output data, consideration must be given to the following factors:

1. Is there a need for central venous or right/left heart pressure data collection to monitor central pressure? If yes, the benefits of pulmonary artery catheter use may outweigh the risks. This is especially important if it is absolutely necessary to monitor pulmonary artery wedge pressures (PAWP). The ICG monitor provides "thoracic fluid content" (by measuring total conductivity of fluid volume in the chest) but these data are not equivalent to PAWP. Thoracic fluid content value reflects both intra- and extra-vascular fluid volumes in the entire chest, unlike a PAWP, which provides information about pulmonary artery diastolic and left ventricular end-systolic volume only.
   
2. Is there a need for a central catheter to deliver medications during an acute, critically ill phase of the illness or during a surgical procedure? If yes, a pulmonary artery catheter may serve both the purpose of delivering medications and providing hemodynamic data.
   
3. Is patient movement or ambulation important to the treatment plan? If yes, an ICG system is ideal because there is no risk of catheter movement or tip displacement with repositioning. If the sensors are displaced with movement, they can be easily replaced (similar to electrocardiogram sensors). In addition, the lead wires attached to the sensors can be disconnected from the monitor during ambulation and easily reattached when the patient returns to bed. These wires are lightweight and can be placed in the chest pocket of a patient’s nightgown.
   

In most emergency, acute, or critical care situations, ICG may serve as an excellent replacement for invasive hemodynamic monitoring. Patients can be monitored continuously or intermittently and data can be used to diagnose, assess prognosis, provide definitive need for new therapies, and assess trends over time. In an outpatient setting, ICG may be a valuable adjunct to care planning when objective and subjective assessment are inconsistent or when trends in hemodynamic values might serve to determine or reinforce treatment strategies.

Nurses must be properly trained in placing sensors in the proper position on the neck and thorax, in understanding the technical information provided on the monitor (eg, thoracic fluid content and the impedance waveform) and in assessing changes in hemodynamic data so that treatment strategies are optimized in an efficient manner. The monitor is easy to set up and program and the risks of invasive hemodynamic monitoring are eliminated.

References

1. Strobeck JE, Silver M. Impedance cardiography: noninvasive measurement of cardiac stroke volume and thoracic fluid content. Congestive Heart Fail. March-April 2000;6: 3–6.
2. Van De Water JM, Miller TW, Vogel RL, Mount BE, Dalton ML. Impedance cardiography: the next vital sign technology? Chest. 2003;123:2028–2033.[Abstract/Free Full Text]
3. Sageman WS, Riffenburgh RH, Spiess BD. Equivalence of bioimpedance and thermodilution in measuring cardiac index after cardiac surgery. J Cardiothorac Vasc Anesthes. 2002;16:8–14.[Medline]
4. Greenberg BH, Hermann DD, Rranulis MF, Lazio L, Cloutier D. Reproducibility of impedance cardiography hemodynamic measures in clinically stable heart failure patients. Congestive Heart Fail. March-April 2000;6:19–26.
5. Drazner MH, Thompson B, Rosenberg PB, et al. Comparison of impedance cardiography with invasive hemodynamic measurements in patients with heart failure secondary to ischemic or nonischemic cardiomyopathy. Am J Cardiol. 2002;89: 993–995.[Medline]
6. Albert NM, Hail MD, Li J, Young JB. Equivalence of bioimpedance and thermodilution in measuring cardiac output and index in patients with advanced, decompensated chronic heart failure hospitalized in critical care [abstract]. J Am Coll Cardiol. 2003;41(suppl A):211A.
7. Ziegler D, Grotti L, Krucke G. Comparison of cardiac output measurements by TEB vs. intermittent bolus thermodilution in mechanical ventilated patients [abstract]. Chest. 1999;116(suppl 2):281S.
8. Connors AF, Speroff T, Dawson NV, et al. The effectiveness of right heart catheterization in the initial care of critically ill patients. JAMA. 1996;18:889–897.
9. Sandham JD. Hull RD, Brant RF, et al. A randomized, controlled trial of the use of pulmonary-artery catheters in high-risk surgical patients. N Engl J Med. 2003;348:5–14.[Abstract/Free Full Text]

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